1. Field of the Invention
This invention relates in general to a semiconductor integrated circuit device and, more particularly, to a semiconductor integrated circuit device in which circuit functions can be remedied or changed. More particularly, it relates to a method for producing such semiconductor integrated circuit device.
2. Description of the Background Art
In a semiconductor integrated circuit, there is known a technique called laser remedying as shown for example in the U.S. Pat. No. 4,641,286, according to which at least one portion of the circuit patterns is severed by irradiation of a laser beam, in order that, when a malfunction occurs in a portion of the circuit on account of defects produced during manufacture or some other reason, the function of the defective circuit may be performed by a built-in redundant circuit.
It is also widely known to sever at least a portion of the circuit pattern of the semiconductor integrated circuit by radiation of the laser beam with a view to changing the function of the semiconductor integrated circuit device.
An example of such severing of the circuit pattern by the laser beam radiation in the MOS type semiconductor integrated circuit, referred to hereinafter as MOS IC, is shown in FIGS. 1A, 1B and 1C. In these figures, that portion of the circuit pattern on the semiconductor substrate 2 of the MOS IC which is assumed to be severed, referred to hereinafter as fuse element 1, is shown to an enlarge scale. FIG. 1A shows the vicinity of the fuse element prior to irradiation of the laser beam. FIG. 1B shows the vicinity of the fuse element after the fuse element is severed by the laser beam in a regular manner. FIG. 1C shows the vicinity of the fuse element when the laser beam is irradiated with a laser beam deviation due to errors or some other reason.
In FIGS. 2A, 2B and 2C, there is shown an example of a fuse element in a MOS IC in which the elements on the MOS IC are electrically separated from one another by a thick oxide film called a LOCOS oxide film. LOCOS is an abbreviation for local oxidation of silicon. FIGS. 2A, 2B and 2C are cross sectional views corresponding to FIGS. 1A, 1B, 1C and taken along line II--II in these figures.
Referring to FIG. 2A, a thick oxide film 3 is formed on the major surface of a P-type semiconductor substrate 4, and a fuse element 1 is formed on this thick oxide film 3. It is well known that, when a laser beam is irradiated with a laser beam deviation, in the LOCOS type MOS IC, no damages are done to the P-semiconductor substrate 4 because the fuse element 1 is formed on the thick oxide film 3. However, in the LOCOS type MOS IC, a so-called bird's beak is formed, as shown at l in FIG. 2A, due to intrusion of the thick oxide film to an unnecessary region, so that a useful area on the P-type semiconductor substrate 4 is taken up wastefully. This represents a considerable hindrance to miniaturization of the circuit of the MOS IC. There is also presented another problem in the LOCOS type MOS IC in that a step difference as shown at h in FIG. 2A is formed on the surface of the thick oxide film 3 and the surface of the P-type semiconductor substrate 4, so that the surface of the MOS IC cannot be flattened to raise difficulties in forming the pattern wiring on the MOS IC surface.
For obviating the above inconvenience, there is also proposed a MOS IC in which the elements on the MOS IC are separated from one another by a gate-separating structure. The theoretical background of such gate-separating structure is explained hereinbelow. When a gate electrode is grounded in an N-type MOS transistor, the source and the drain of the N-type MOS transistor are electrically separated from each other. Similarly, when a potential exceeding the threshold voltage is applied to the gate electrode of the P-type MOS transistor, the source and the drain of the P-type MOS transistor are electrically separated from each other. Hence, in a MOS IC, a layer of an electrically conductive material, called a field shield plate, having a structure similar to that of the gate electrode of the MOS transistor, may be formed in regions of the MOS IC other than regions reserved for capacitors or active elements, such as MOS transistors, and a potential can be applied to this field shield plate to effect the electrical separation of the device of the MOS IC. Such structure is called the gate-separating structure and disclosed in, for example, the Japanese Patent Laying-Open No. 162353/1987.
FIG. 3 is partial plan view of the MOS IC having the gate-separating structure. FIG. 4 is a cross-sectional view taken along line IV--IV in FIG. 3. Referring to these figures, regions 6 for active elements, in which capacitors or active elements such as MOS transistor 21, are to be formed subsequently, are formed on the major surface of the P-type semiconductor substrate. The regions other than the regions 6 for active elements are covered by a field shield plate 5. The field shield plate 5 is a layer of an electrically conductive material for electrically separating the elements from one another. A pattern wiring 1a and a fuse element 1 are formed on the field shield plate 5 by way of an insulating film 7. The field shield plate 5 is grounded. It is noted that, in FIG. 3, the elements such as MOS transistors or the pattern wiring other than pattern wiring 1a are not shown for clarity.
An example of servering of the fuse element 1 by the laser beam irradiation in FIG. 3 is shown in FIGS. 5A, 5B and 5C which are cross-sectional views corresponding to FIG. 1A, FIG. 1B and 1C respectively. Referring to these figures, a negative potential VBB is applied to the P-type semiconductor substrate 4, while the field shield plate 5 is grounded. With this system, defects such as those shown in FIG. 2A by l or h may be eliminated. However, the sum of the thickness of the filed shield plate 5 and that of the insulating film 7 is less than the thickness of the thick oxide film 3. Hence, as shown in FIG. 5C, when the laser beam is irradiated with a laser beam deviation, the field shield plate 5 and the P-type semiconductor substrate 4 are short-circuited to result in short-circuiting of the ground potential and the VBB potential. Therefore, the system is poor in operation reliability structurally. Meanwhile, in such MOS IC having the gate separating structure, when the thick oxide film 3 is also provided on a portion of the MOS IC, the fuse element 1 may be provided on this thick oxide film 3 to avoid the above problem. However, in such a case, the fuse element 1 may be provided only at restricted portions on the LOCOS oxide film so that limitations are necessarily imposed on the layout of the circuit pattern.